The ability to separate a particle/fluid mixture into its separate components is desirable in many applications. Physical size exclusion filters can be used for this purpose, where the particles are trapped on the filter and the fluid flows through the filter. Examples of physical filters include those that operate by tangential flow filtration, depth flow filtration, hollow fiber filtration, and centrifugation. However, physical filters can be complicated to work with. For instance, as the physical filter fills up, filtration capacity is reduced. Also, using such filters requires periodic stopping to remove the filter and obtain or clear the particles trapped thereon.
Acoustophoresis is the separation of particles using high intensity sound waves, and without the use of membranes or physical size exclusion filters. It has been known that high intensity standing waves of sound can exert forces on particles. A standing wave has a pressure profile which appears to “stand” still in time. The pressure profile in a standing wave contains areas of net zero pressure at its nodes and anti-nodes. Depending on the density and compressibility of the particles, they will be trapped at the nodes or anti-nodes of the standing wave. However, conventional acoustophoresis devices have had limited efficacy due to several factors including heat generation, limits on fluid flow, and the inability to capture different types of materials. Improved acoustophoresis devices using improved fluid dynamics would be desirable.